The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted to be prior art against the present disclosure.
A delay-locked loop (DLL) may be used to lock a signal to a reference signal—i.e., to generate an output signal that has a constant delay relative to the phase of the input reference signal. In a basic delay-locked loop, the output of a variable delay chain (i.e., a chain of variable delays) is looped back to the input of a phase detector, which also has a reference signal as another input. The phase detector examines the phase difference between the looped delay chain output and the reference signal, and generates a control signal that adjusts the variable delays in the delay chain to align the phase of the delay chain output with the phase of the reference signal. Typically, the phase detector output merely indicates whether the delay chain output is ahead of or behind the reference signal and therefore whether the delay chain output should be retarded or advanced.
A multiplying delay-locked loop (MDLL) similarly uses a chain of variable delays, but the pulse edge of the reference signal (herein referred to as a “reference edge”) is used to replace the edge of a free-running oscillator (e.g., a ring oscillator) within the DLL (herein referred to as a “free-running edge”) at selected intervals (typically the selected interval corresponds to each period of the reference signal, which is normally longer than the period specified by the free-running oscillation frequency of the ring-oscillator—herein referred to as the “free-running period”—unless the multiplier N=1).
Replacing the edge of the MDLL output signal causes a perturbation of the output signal, as a rising or falling edge is moved from where it otherwise would have fallen. Such a perturbation is referred to as a “spur.”